The present invention concerns the encoding, transmitting and decoding of digital information and more particularly, using conventional audio transmitting or recording media in the transmission of digital data.
While transmission of information or data is not a recent development, an ever-increasing variety of particularly suitable methods by which the data is transmitted are continually being developed. Most often, the data, which are desired to be in digital form, are arranged to be provided as binary signals, that is "1" or "0". The method of transmission utilized is dependent upon a number of factors including the intended use of the information, the density of the information signal, and information accuracy required.
In most cases a clock signal is necessary to transmit the information, requiring that the clock signal be reconstructed either independently or extracted from the transmitted signal when decoding is desired. The transmission may be unipolar or bipolar. The terms unipolar and bipolar are utilized to refer to a particular characteristic of the signal waveform. The signal comprises a series of pulses the lengths of which are determined by characteristics of circuitry used to obtain the signals. The signal has a reference line, usually representing zero potential or current, from which the series of pulses originate. Unipolar pulses extend in only one direction from the reference line, while bipolar pulses extend in both directions.
The information signal may be single or multiple leveled, that is pulses may have more than one distinct maximum value representing different information conditions. The information may be represented by single or multiple tracks of signals.
Examples of data signals which have been frequency modulated for transmission are illustrated in U.S. Pat. Nos. 3,049,698 and 3,218,618.
U.S. Pat. No. 3,786,201 utilizes two separate digital signals (multiple tracks) for each information bit to be transmitted. The information is sampled, gain adjusted and converted into an eight bit digital signal. Two ranging bits, representing the overall gain of the sampled information, are combined with the eight bit signal to form the two tracks transmitted signal.
In high density data transmission situations, multilevel zero average singals have been utilized in systems illustrated in U.S. Pat. Nos. 3,921,210 and 4,020,282. The encoded signals actually occupy longer time intervals than the corresponding information to be encoded.
Multiplex recording of digital information and a television signal is taught in U.S. Pat. No. 3,732,364. The horizontal synchronization portion of the television signal it utilized as a clock signal to record the digital information. In order to avoid confusion with the television signal during transmission and receiving, the digital information is modulated at a frequency outside television frequency ranges.
Converting unipolar information signals into bipolar zero referenced signals, referred to as "return to zero" signals, is illustrated in U.S. Pat. No. 3,609,755. The conversion is accomplished through reflection in a short circuited matched line.
A clock signal is often utilized to encode the information for transmission. Once the information has been transmitted, the receiving system requires an identical clock signal to decode the information. Rather than remanufacture the clock signal, the simplest method is to extract the clock signal from the received signal. U.S. Pat. No. 3,518,700 teaches one such system. A binary information signal is converted into a transmission signal having a clock portion, a data portion and a high frequency portion. The high frequency portion ensures elimination of all previously recorded signals on the recording medium.
A scheme for encoding digital information is shown in U.S. Pat. No. 3,646,534. Each of two possible binary bits is represented by a different position of unipolar clock pulses.
U.S. Pat. No. 2,954,267 teaches recording and transmitting digital information encoded as a return to zero, bipolar signal. A series of negative pulses are interlaced with the information. The series of negative pulses are utilized as a clock signal for timing. The reference uses the negative pulses to ensure cancellation in the magnetic recording material immediately prior to the recording of each information bit.
A bipolar return to zero signal including information pulses and a clock signal is shown in U.S. Pat. No. 3,108,265. The clock signal is bipolar. The information pulses, which are likewise bipolar, are positioned intermediate the pulses of the clock signal. One information condition, no signal, is represented by the lack of an information pulse following a clock pulse. The other information condition is represented by a series of pulses identical to a clock pulse immediately following the pulse of the clock signal.
In U.S. Pat. No. 3,863,025 a bipolar clock signal is utilized as a carrier signal for the information. The clock is a repetition of a negative pulse immediately followed by a positive pulse. The binary information signal is encoded for recording and/or transmitting by inverting the clock signal when a binary "one" bit is present at the same time as a pulse of the clock signal.
The foregoing references concern the encoding of binary information signals for subsequent recording and/or transmitting. As will be apparent, the methods by which the binary information is encoded are as varied as are the uses of the encoded binary information. The present invention arises out of the need for a system to control a number of remotely located system components from a centralized system control station. Control of the remote system components is effected through a digital signal generated with a series of electrical switches which relate to individual remote components and the particular functions intended to be performed by those components.